(1) Field of the Invention
The present invention relates to a pulse-width controller suitable for use in an electronic apparatus which uses pulses; e.g., a digital circuit, a pulse-width modulation circuit, or a light-emitting element drive circuit.
(2) Description of the Related Art
In an optical communications system or an optical disk system, control is necessary to ensure a constant pulse width as well as a constant pulse amplitude in order to reduce an error rate associated with communication and writing of data.
FIG. 16 is a schematic representation illustrating the operation of a common pulse-width controller. The pulse-width controller illustrated in FIG. 16 is provided with a pulse-width adjusting section 30. This pulse-width adjusting section 30 adjusts the pulse width of input data according to a control signal input from a control terminal; namely, d.c. voltage (DC voltage), d.c. current (DC current), or a resistance connected to the control terminal. Upon reception of a data signal, the data pulse-width adjusting section 30 outputs, according to a control signal input from the control terminal, a data signal having a target pulse width.
However, the characteristics of components constituting the previously described common pulse-width controller vary according to temperature and voltage of the power source. Moreover, they also vary depending on the individual difference of the components; for example, the individual difference of an IC (integrated circuit) or LSI (large scale integrated circuit) due to the influence of the degree of reproducibility of a process and the uniformity of the surface of a wafer.
In a case where a pulse-width controller is constituted of such circuit components, even if a DC current or DC voltage input to the control terminal of the pulse-width adjusting section 30 or a resistance connected to the control terminal of the same is maintained constant, the pulse width of resultantly-output data signal changes with factors such as the environment (conditions 1 and 2) when the pulse-width control is performed as designated by arrows A and B in FIG. 16. As a result, it becomes impossible to precisely control the pulse width.
Further, as shown in FIG. 17, it is also conceivable that, in a pulse-width control apparatus comprising the pulse-width adjusting section 30 followed by an electrical-optical signal converter (E/O signal converter) 32 which converts an electrical signal to an optical signal, the output received from the pulse-width adjusting section 30 is compared with a predetermined reference value by a comparator 31. The pulse-width adjusting section 30 adjusts the pulse width of output data according to the result of such comparison, thereby making the pulse width stable.
With such a circuit configuration, it is possible to maintain the pulse width constant in a case where a data signal is continuously input. However, in the case of a burst mode signal (e.g., communications employed with respect to optical subscribers of CATV), a data signal is input intermittently; more specifically, there arises a difference between the state in which input of the data signal is interrupted for a given period of time and the state in which there is an input of the data signal, thereby making it impossible to produce a stable pulse width.